The invention relates to an apparatus and method for the displacement impregnation of cellulosic chips material with digesting liquid. By effecting displacement impregnation of cellulosic ships material according to the present invention, the energy consumption during digestion is greatly reduced over prior art practices, the amount of scaling is reduced, the amount of turpentine and methanol that can be economically recovered is enhanced, and chemical use can be reduced.
According to the present invention, countercurrent impregnation is established in the impregnation zone of a treatment vessel and moisture, calcium, and other materials displaced from chips material in the impregnation zone are withdrawn through the topmost portion of the treatment vessel, into the feed system for the vessel. This is distinct from the prior art, as exemplified by U.S. Pat. Nos. 3,802,956 and 3,097,987, wherein there is no liquid withdrawal into the feed system, and wherein only partial displacement impregnation takes place. In U.S. Pat. No. 3,097,987, only liquid displaced from the introduced chips liquid mixture is separated through outlet 45, the impregnation taking place co-currently and the impregnation liquid being removed through outlet 21. Even in the pre-hydrolysis embodiment thereof, (FIG. 2), liquid is withdrawn through the outlet 47 after some countercurrent flow in zone 10, and no liquid is withdrawn from the impregnation zone through the outlet 45 at the top of the treatment vessel into the feed system, as according to the present invention. In U.S. Pat. No. 3,802,956, liquid is withdrawn from the impregnation zone through screens 28, and usually only liquid separated from the transport system is separated through strainer girdle 25. Even if the strainer 28 is eliminated, the liquid is withdrawn through line 30, and not the feed system. Additionally, according to the present invention, a chips plug is established at the top of the treatment vessel above the level of the chips column in the vessel, the chips plug isolating the feeder from the impregnation zone and providing for straining of all of the liquid being withdrawn from the vessel to keep some products that are separated from the chips within the impregnation zone so that they do not contaminate the feed system.
Displacement impregnation according to the present invention has many advantages over the prior art devices; as mentioned above, energy consumption is reduced, scaling is reduced, chemical use is reduced, and the amount of turpentine and methanol recovered is increased. By displacing all of the water in the chips before the chips enter the cooking zone, according to the present invention the "cold" associated therewith is also displaced. The water essentially never enters the cooking zone, but rather is withdrawn through the feed system, and the energy requirements for digesting the chips are thus greatly reduced. At the same percentage chemical application, the concentration is increased according to the invention over the prior art, since the water in the chips is displaced and, therefore, the cooking temperature can be reduced with equal treatment. Also, since no strainers are provided that can clog and provide "channeling" in the impregnation zone, the treatment is completely uniform (all the withdrawn liquid passes through the chips plug at the top of the treatment vessel). The mass of the chips is reduced, since the water is displaced before reaching the cooking zone, therefore, the exothermic heat from the digestion reactions provides a greater percentage of the energy requirements for digestion.
Along with the displaced water from the chips goes calcium other minerals, the calcium and other minerals being displaced before the chips enter the high temperatures of the cooking zone (and, as mentioned above, the temperatures of the cooking zone may be less). Since the calcium has a tendency to react with the carbonate in the white liquor at the high temperatures in the digestion zone with resulting scaling of the screens, heaters, etc., therein with less calcium and the like present in the cooking zone, the scaling is reduced. Also, since the extraction is in the feed system, the methanol and turpentine can be removed from lines operatively connected to the feed system with a resulting higher percentage recovery (since black liquor, with high solids content is not separated therewith). The turpentine is separated by a conventional turpentine decanter, while the methanol is separated by a conventional fractional distillation tower. Also, if sand is present in the feed system it can be removed without ever having entered the actual treatment parts of the treatment vessel.
Since the removal of many minerals, etc. takes place in the feed system, the pH of the feed system may reach a level where resin build up and the like takes place in the component parts. This may be avoided according to the present invention by taking white liquor and feeding it into the end bell portions of the high pressure feeder. Also, according to the present invention, the displaced liquid can be utilized in other portions of the digestion system, resulting in a savings in ultimate material usage.
While according to the present invention, the impregnation zone can be in a separate vessel and connected to a digester through a vapor zone (as in the U.S. Pat. No. 3,802,956), it is preferred that there be a hydraulic connection between the cooking and impregnation zones, whether they be in separate hydaulically connected vessels, or in the same vessel. This allows liquid from the cooking zone to be drawn into the impregnation zone as needed, and thus more digesting liquid can be added than is displaced, and a better control of the treatment processes is effected.
According to the present invention, apparatus for treating cellulosic chips material is provided comprising a source of cellulosic chips material and liquid, a high-pressure transfer valve for transporting the cellulosic chips and liquid under pressure, and a high pressure vertical treatment vessel having at least an impregnation zone in an upper portion thereof, and a topmost portion above the impregnation zone. A chips and liquid inlet to the vessel and a liquid withdrawal outlet from the vessel are provided in the topmost portion of the vessel, connected up to a feed system from and to the high-pressure transfer valve. Means are also provided for establishing a countercurrent flow of digesting liquid in the impregnation zone of the vessel for impregnation of the chips material therein with digesting liquid, said means including means for withdrawing liquid from the impregnation zone through the topmost portion of the vessel into the feed system. Means are provided at the bottom of the vessel for withdrawing treated chips material. Means are provided for establishing a chips plug in the vessel below the inlet and outlet, and such means for establishing a chips plug for isolation of the topmost portion may include a screw feeder rotatable about a vertical axis and surrounded by a tubular screen, the inlet to the topmost portion of the vessel being inside the tubular screen, and the outlet from the topmost portion of the vessel being outside the tubular screen, and a solid wall, generally tubular member, extending downwardly from the bottom of the screw feeder and said tubular screen for capturing chips therein and forming the chips plug. Digestion and washing may also take place in the same vessel in which the impregnation zone is located, or impregnation may take place in a separate vessel connected hydraulically to one or more other vessels for digestion and washing of the chips. Means may be provided for feeding digesting liquid to an end portion of the high pressure transfer valve for maintaining the pH of the liquid surrounding the valve at 8 or above.
According to the method of the present invention, utilizing a high pressure transfer valve and at least one vertical treatment vessel having a topmost portion and an impregnation zone in an upper portion thereof below the topmost portion, a source of cellulosic chips material and liquid is established, and the chips material and liquid are fed to the high pressure valve to boost the pressure thereof. The high pressure chips material and liquid are -- through a feed system -- transported to the topmost portion of the vertical treatment vessel and introduced into the topmost portion thereof, while at the same time, liquid separated from the chips is withdrawn from the impregnation zone into the topmost portion of the vessel into the feed system.
A chips plug is established at the topmost portion of the vessel below the withdrawal point of liquid therefrom and below the inlet of the chips thereto, the chips plug providing isolation of the topmost portion of the vessel from the impregnation zone.
A countercurrent flow of digesting liquid is established in the vessel impregnation zone to displace the minerals and water from the chips material and impregnate them with digesting liquid, and treated chips material is withdrawn from the bottom of the vessel. Sand may be separated from liquid in a line operatively connected to the high-pressure transfer valve by high efficiency centrifuging thereof.
It is the primary object of the present invention to provide a method and apparatus for the displacement impregnation of chips material, reduced energy requirements and scaling, and eliminate other problems encountered in prior art apparatus. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.